Re: [Python-ideas] Adding bytes.frombuffer() constructor

On Mon, Aug 8, 2016 at 12:47 AM, Nick Coghlan wrote:

On 7 August 2016 at 15:08, Michael Selik wrote:

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On Sat, Aug 6, 2016 at 8:45 PM INADA Naoki wrote:

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1. bytes(bytearray[:n]) 2. bytes(memoryview(bytearray)[:n])

(1) is simplest, but it produces temporary bytearray having n bytes.

Does that actually make the difference between unacceptably inefficient performance and acceptably efficient for an application you're working on?

Yes. My intention is Tornado and AsyncIO. Since they are framework, we can't assume how large it is -- it may be few bytes ~ few giga bytes.

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While (2) is more efficient than (1), it uses still temporary memoryview object, and it looks bit tricky.

Using the memoryview is nicely explicit whereas ``bytes.frombuffer`` could be creating a temporary bytearray as part of its construction.

It could, but it wouldn't (since that would be pointlessly inefficient).

The main question to be answered here would be whether adding a dedicated spelling for "bytes(memoryview(bytearray)[:n])" actually smooths out the learning curve for memoryview in general, where folks would learn:

1. "bytes(mybytearray[:n])" copies the data twice for no good reason 2. "bytes.frombuffer(mybytearray, n)" avoids the double copy 3. "bytes(memoryview(mybytearray)[:n])" generalises to arbitrary slices

With memoryview being a builtin, I'm not sure that argument can be made successfully - the transformation in going from step 1 direct to step 3 is just "wrap the original object with memoryview before slicing to avoid the double copy", and that's no more complicated than using a different constructor method.

I'm not sure, too. memoryview may and may not be bytes-like object which os.write or socket.send accepts. But memoryview is successor of buffer. So we should encourage to use it for zero copy slicing. Thank you. -- INADA Naoki